ESTRO 36 Abstract Book

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S866 ESTRO 36 2017
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neck personalized Accuform cushions (CIVCO) were used.
Initial repositioning using CBCT before every fraction and
6DoF corrections was performed. Post-treatment CBCT
repositioning was also performed to assess intra-fractional
motion. Positional errors in all six directions were
compiled in addition to 3D total vector errors (TVE).
Results
Intrafraction translation errors (cm) for the vertical,
cranio-caudal and lateral directions were: 0.01 ± 0.03,
0.02 ± 0.05 and -0.01 ± 0.04 respectively (Mean± SD). 3D
TVE was 0.07 ± 0.04(Mean± SD). Intrafraction rotational
errors for pitch, roll and rotation were 0.13 0 ± 0.330, 0.040
±0.180 and -0.030 ± 0.350 respectively(Mean± SD) (Table.1).
Boxplots presented in Figure.1 show a small variability of
the TVE with a range of errors when we eliminate outliers
of 0.12cm; however 92% of the cases were within 0.1cm
deviation.
Conclusion
The use of personalized single layer masks with custom
made Accuform cushions produces stable positioning for
use on TrueBeam with Perfect Pitch platforms.
Intrafraction motion showed a mean TVE of 0.07 ± 0.04cm.
These results suggest that a PTV margin of 0.1cm for SRS
cases and 0.2cm for SRT cases is justified to mitigate
intrafraction errors.
EP-1622 Intra-fractional isocenter position analysis and
dose evaluation of DIBH using surface guided RT
L. Berg1, M. Kügele 1,2, A. Edvardsson 2, S. Alkner1, C.
Andersson-Ljus1, S. Ceberg1
1 Skåne University Hospital, Department of hematology
oncology and radiation physics, Lund, Sweden
2 Lund University, Medical Radiation Physics - Department
of Clinical Sciences, Lund, Sweden
Purpose or Objective
The use of surface-guided radiotherapy (SGRT) for deep
inspiration breath hold (DIBH) was investigated. Cardiac
and pulmonary dose-volume indices were compared during
free breathing (FB) and DIBH for left-sided breast cancer
patients. In this study, we calculated intra-fractional
isocenter shifts based on surface scanning for the first
time to investigate potential breathing variations during
beam delivery for the individual patient.
Material and Methods
Twenty patients treated with tangential technique, SGRT
and DIBH were included. They underwent two computed
tomography (CT) scans; one during FB and one during
DIBH, which enabled a dose planning study evaluating
possible dose sparing with SGRT and DIBH. Target volumes
and risk organs were contoured by the same physician in
both scans. Individual treatment plans were created and
dose-volume indices for the heart, the left anterior
descending artery (LAD) and the ipsilateral lung were
evaluated.
The optical scanning (OS) systems Sentinel and Catalyst
HD (C-RAD positioning AB, Sweden) were used at CT and
treatment, respectively. At CT the breathing motion was
monitored using an optical tracking point on the skin
above the xiphoid process. The size of the gating window
was 3 mm and the amplitude of the breath hold was
individual for each patient. At treatment the OS system
was used for both patient positioning and DIBH delivery.
The irradiation was controlled using the tracking point and
with the same amplitude and gating window determined
during the CT session.
Retrospectively, the coordinates of the calculated
isocenter according to the OS system during beam-on was
used to investigate intra-fractional motion in between the
two separate DIBHs during beam delivery of the two
tangential fields. The difference in isocenter position was
evaluated for 190 DIBHs from randomly selected
treatment fractions.
Results
The mean DIBH amplitude was 10.5±2.8 (1 SD) mm. The
mean dose for the heart was reduced from 1.5±0.8 Gy for
FB to 0.8±0.3 Gy for DIBH, and for the lung from 5.9±1.4
Gy for FB to 5.5±1.5 Gy for DIBH. Dose sparing was also
seen for LAD where the mean dose was 9.6±7.0 Gy for FB
and 3.8±2.9 Gy for DIBH. The maximum doses, represented
as D 2%, were reduced from 14.4±15.2 Gy for FB to 3.6±2.7
Gy for DIBH and from 29.0±18.9 Gy for FB to 10.8±12.3 Gy
for DIBH for the heart and LAD, respectively. The intra-
fractional motion of the isocenter between two DIBHs was
small and the median values were 1.3 mm, 1.2 mm and
0.9 mm in the lateral, longitudinal and vertical directions,
respectively (Figure 1).
Conclusion
For the first time, optical surface scanning was used to
evaluate isocenter motion during irradiation. The median
intra-fraction motion of the isocenter in the breast during
beam-on was less than 1.3 mm in all directions, using a
tracking point above xiphoid process and a 3 mm gating
window. It was shown, within this study, that the use of
SGRT during DIBH for left-sided breast cancer patients
results in decreased doses to organs at risk .
EP-1623 SeedTracker: Enabling real time position
monitoring with a conventional linacs for prostate SBRT
A. Sankar1,2, L. Holloway1,3,4,5, D. Truant6, A. Xing7,8, L.
Karen8,9, A. Walis10, M. Grand11, M. Sidhom8,9
1Liverpool and Macarthur cancer therapy centres and
Ingham Institute, Department of Medical Physics,
Sydney-New South Wales, Australia
2University of New South Wales, South Western Sydney
Clinical School’, Sydney-New South Wales, Australia
3University of Wollongong, Centre for Medical Radiation